make the lxc backend option conditional on lxc version
make the lxc backend option conditional on lxc version

The lxc backend store configuration option is deprecated in lxc3. This
change ensures that the option is no longer rendered in the lxc template
when the version of lxc is greater than or equal to version 3.

Change-Id: I207ed1f89604979e74667ae4c603d36304a6ed53
Signed-off-by: Kevin Carter <ffb4761cba839470133bee36aeb139f58d7dbaa9@cloudnull.com>

openstack/openstack-ansible-lxc_hosts,openstack/openstack-ansible-lxc_hosts